1. Field of the Invention
This invention relates to an electron source and an image forming apparatus and, more particularly, it relates to an electron source provided with means for maintaining it in an activated state by suppressing degradation of and restoring the performance thereof and an image forming apparatus comprising such an electron source as well as a method of providing it with such means.
2. Related Background Art
There have been known two types of electron-emitting device: the thermionic cathode type and the cold cathode type. Of these, the cold cathode refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices. Examples of FE type devices include tho se proposed by W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "Physical Properties of thinfilm field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5284 (1976).
Examples of MIM device are disclosed in papers including C. A. Mead, "The tunnel-emission amplifier", J. Appl. Phys., 32, 646 (1961).
Examples of surface conduction electron-emitting device include one proposed by M. I . Elinson, Radio Eng. Electron Phys., 10, 1290 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO.sub.2 thin film for a device of this type, the use of Au thin film is proposed in G. Dittmer: "Thin Solid Films", 9, 317 (1972) whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 and that of carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975) and H. Araki et al.: "Vacuum", Vol. 26, No. 1, p.22 (1983).
FIG. 27 of the accompanying drawings schematically illustrates a typical surface conduction electron emitting device proposed by M. Hartwell. In FIG. 27, reference numeral 1 denotes a substrate. Reference numeral 4 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 5 when it is subjected to an electrically energizing process referred to as "energization forming" as described hereinafter. In FIG. 27, the thin horizontal area of the metal oxide film separating a pair of device electrodes has a length L of 0.5 to 1 mm and a width W' of 0.1 mm.
Conventionally, an electron-emitting region 5 is produced in a surface conduction electron-emitting device by subjecting the electroconductive thin film 4 of the device to an electrically energizing preliminary process, which is referred to as "energization forming". In the energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1 V/min. is applied to given opposite ends of the electroconductive thin film 4 to partly destroy, deform or transform the film and produce an electron-emitting region 5 which is electrically highly resistive. Thus, the electron-emitting region 5 is part of the electroconductive thin film 4 that typically contains a fissure and fissures therein so that electrons may be emitted from the fissure.
Currently available electron-emitting devices of the type under consideration have room for improvement in terms of performance and efficiency of electron emission in order to realize image forming apparatuses that provide bright and clear images on a stable basis. The efficiency here refers to the ratio of the electric current flowing through the surface conduction electron-emitting device (hereinafter referred to as "device current" or If) to the electric current formed by electrons discharged from the device into vacuum (hereinafter referred to as "emission current" or Ie) when a voltage is applied to the paired device electrodes of the device. An ideal electron-emitting device will show a large emission current relative to a small device current. If an electron-emitting device is rigorously controllable for its electron emitting performance and has an improved efficiency, an image forming apparatus realized by arranging a number of such electron-emitting devices and a fluorescent member for forming images thereon will be able to produce high quality images with a reduced energy consumption rate if the apparatus is made very flat. Then, the drive circuit of such an image forming apparatus can be manufactured at reduced cost because of the low energy consumption rate of the electron-emitting devices of the apparatus.
However, Hartwell's electron-emitting device does not necessarily perform satisfactorily in terms of stable emission of electrons and efficiency and, therefore, it is thought to be very difficult to realize an image forming apparatus that operates stably to produce highly bright images with Hartwell's devices.
As a result of intensive research efforts, the inventors of the present invention discovered that, if a certain voltage is applied to a surface conduction electron-emitting device in an atmosphere that contains organic substances after producing an electron-emitting region therein by energization forming as described above, both If and Ie of the device remarkably increase. This operation of applying a certain voltage is termed "activation".
The above phenomenon of increased If and Ie is attributable to an activated filmy deposit of carbon or a carbon compound produced in the vicinity of the electron-emitting region as a result of the voltage application.
As an electron-emitting device is operated for a long time for electron emission, the deposit in the vicinity of the electron-emitting region may be gradually decomposed and eroded to degrade the electron-emitting performance of the device, although such degradation may be suppressed by selecting appropriate parameters for the activation process. This may be because the crystallinity of the deposit affects the rate of erosion and the crystallinity is by turn affected by the parameters of the activation process. The use of a metal having a high melting point such as tungsten for the deposit is effective for suppressing the erosion of the deposit.
Nevertheless, the performance of a surface conduction electron-emitting device has to be further improved in order to pr event degradation and prolong its service life if it is to be used in an image forming apparatus or a similar application.
In view of the above identified problems and other problems, it is therefore an object of the present invention to provide an improved surface conduction electron-emitting device.
Additionally, the "activation process" requires the use of a large vacuum apparatus provided with equipment for introducing carbon and/or metal compounds into the apparatus. When a large image forming apparatus having a vacuum container (envelope) is subjected to an activation process with such a vacuum apparatus, the latter has to be provided with an exhaust pipe for evacuating the inside of the vacuum container and introducing carbon and/or metal compounds into the vacuum container to make the overall operation rather complicate d and time consumin g to push up the manufacturing cost of the image forming apparatus particularly if such compounds have a large molecular weight. Thus, the present invention is also intended to provide a method that allows the use of a downsized vacuum apparatus and a simplified manufacturing process to bypass the above problems.